This invention relates generally to cold seal release films, and more particularly to packaging structures employing. Cold seal release films having improved scuff resistance.
Cold seal release films are most commonly used as the outside web or film of a multi-web packaging structure. In common practice, the release film is a clear web that is reverse printed and adhesive laminated to an inner web such as an opaque film (in a candy bar wrap) or a metallized film (in a breakfast bar wrap). After the two films are laminated a layer of cold seal adhesive is applied to the inner web as the web is wound into a roll. The cold seal release film serves the following functions: 1) it provides a transparent web which can be reverse printed so that the decoration is “buried” beneath the clear web to prevent the ink from being worn away; 2) it provides the desired slip properties so that the laminate will run effectively on packaging machines and 3) it provides a surface with low adhesion to the cold seal adhesive so that the laminate roll can be unwound during a packaging operation, with the adhesive remaining on the desired surface of the lamination.
The most successful prior art cold seal release film in the market place today employs a polypropylene homopolymer as the cold seal release surface. This prior art film provides good printing characteristics, low coefficient of friction (COF) and a reasonably low adhesive release force from the most common cold seal adhesives; namely, acrylic/rubber latex adhesives. Moreover, prior art cold release films employing a polypropylene homopolymer also have reasonably good scuff resistence. However, the prior art films employing polypropylene homopolymer have a major deficiency; namely, they do not provide the desired low adhesive release force from synthetic adhesives, e.g., ethylene vinyl acetate based adhesives.
Other polymer films containing 1-butene homopolymer, 1-butene/ethylene copolymers (collectively referred to as polybutenes) and 1-butene copolymers blended with ethylene and/or butene random polypropylene copolymers yield low adhesive release force from both synthetic adhesives and acrylic/rubber latex adhesives. However, these prior art films do not have desired scuff resistance properties. Representative patents disclosing these latter polymer compositions for cold release films are U.S. Pat. Nos. 5,981,047; 5,798,174; 5,489,473 and 5,792,549, which are all incorporated herein by reference.
As noted above, an important characteristic for cold seal release films is good scuff resistence. This is an important property because the film serves as the outside web or layer in a monolayer or multilayer cold seal packaging application. Therefore, scuffing, which can occur either during the printing/laminating process or the packaging process, causes the finished product to have a dull or shop-worn appearance, which is undesirable.
The following additional patents also disclose films employing butene polymers:
U.S. Pat. No. 4,554,321 to Hwo et al. discloses heat sealable films having improved seal impact strength comprising a stereoregular, elastomeric polymer of butene-1 or blends thereof with conventional butene-1-ethylene copolymers, optionally with small amounts of polypropylene and/or crystallization nucleating agents. The use of such stereoregular, elastomeric polymers is not desirable for use in cold seal release films of this invention.
U.S. Pat. No. 5,998,039 to Tanizaki, et al. discloses a polypropylene composite film comprising (1) a crystalline polypropylene layer and (2) a layer formed from a blend of a polypropylene homopolymer (5-95%) and a propylene 1-butene random copolymer (95-5%), wherein the propylene component is greater than 50%, by weight, of the copolymer composition.
WO 00/32395 A1 to Chang et al. discloses a polypropylene film containing a non-migratory slip and release additive package of a synthetic or mineral wax, an aluminosilicate additive, and optionally silicone oil and a cross-linked silicone polymer resin to improve cold seal adhesive release properties. Although this patent discloses adding nucleating agents to a polypropylene film for improving the strength of the film, there is no disclosure or suggestion that nucleating agents should or could be added to a polymer composition including a poly-1 butene homopolymer or copolymer composition.
Nucleating agents are known to increase the stiffness of processed materials. Nucleating agents also are known to improve the optical and barrier properties of the resulting materials. Sodium benzoate is a desirable nucleating agent because, for among other reasons, it is readily available and economically practical to use.
However, the use of nucleating agents such as sodium benzoate in the production of biaxially oriented polypropylene films has been considered to be undesirable because, in the amounts typically employed, it creates processing difficulties. U.S. Pat. No. 5,837,177 to DiNardo et al. discloses a method of making a polypropylene product for use in making biaxially oriented films. The method includes the addition of an improved nucleating system that allows for even distribution of a nucleating agent in a polypropylene material. The improved nucleating system comprises a nucleating agent such as sodium benzoate and a liquid miscible with the nucleating agent. The solution is combined with the polypropylene material, and the miscible liquid is removed during processing.
There is a need in the packaging field for an improved cold seal release film exhibiting both a low adhesion to cold seal adhesives, including synthetic adhesives, and improved scuff resistance. It is to such films that the present invention relates.
All references cited herein are incorporated herein by reference in their entireties.